Decadal Shift in the Relationship between the Winter Arctic Oscillation and Indian Ocean Precipitation during the Early 2000s

The present study investigated the long-term change in the interannual relationship between the boreal winter Arctic Oscillation (AO) and tropical Indian Ocean (TIO) climate during 1979–2019 and found that their linkage experienced a decadal change in 2001/2002. The 19-yr sliding correlation coefficients between the January–February–March AO index and central TIO (0–10°S, 65°–80°E) precipitation were significant and varied between approximately 0.5 and 0.7 during 1979–2001, while their correlations abruptly decreased to below 0.4 after 2001 and reached minimum values below 0.2 after 2007. At the same time, the spatial patterns of the AO-related atmospheric circulation anomalies also displayed different features. The anomalous anticyclonic circulation in the middle troposphere over the Arabian Sea moved northwestward and strengthened during 2002–2019. The location of the anomalous anticyclone was at approximately 14°N and 60°E in 1979–2001 and at 17°N and 56°E in 2002–2019. In addition, the correlation coefficient between the JFM AO index and mean vorticity of the anticyclone center was −0.38 before 2001 and −0.63 after 2001. Correspondingly, there were significant southward cross-equator air flows between approximately 65° and 75°E during 1979–2001 that enhanced the intertropical convergence zone and upward air motions, leading to more precipitation over the central TIO. During 2002–2019, evident cross-equator air flows were observed in the western TIO (40°–50°E). An analysis shows that the AO may modulate the Arabian anticyclone through two Rossby wave paths in the upper troposphere. During 1979–2001, the AO-associated wave activity fluxes reached a maximum in the North Atlantic between 50° and 60°N and traveled southeastward to the Middle East and neighboring Arabian Sea. In contrast, during 2002–2019, wave activity fluxes propagated downstream from the North Atlantic to the Middle East and Arabian Sea along 20°–30°N. The mechanisms causing these decadal changes in AO-associated wave trains need further investigation.